For today's applications, high requirements are imposed on colour display systems such as projection displays. As the human eye is very sensitive to colour changes, it is an essential feature to obtain an optimum colour display. This includes both a high contrast and a good colour homogeneity, i.e. a reduced colour shift as well as a high colour purity, i.e. whereby the different colours generated in the colour display matches predetermined colour points as good as possible.
To obtain colour display and/or projection systems basically three different types of projection systems are known: a single imager spatial colour filter design, whereby each pixel is divided into three colour sub-pixels; a single imager colour field sequential system, whereby the beam is sequentially filtered into each primary colour which is then modulated in the single imager, and a two, three or multi-imager parallel colour system, whereby two, three or more light beams of basic colours each are directed onto a separate imager and the modulated colour beams are converged into a single beam for projection. As the latter system allows the highest throughput efficiency, i.e. higher than three times the throughput efficiency of the other systems, the use of a three-imager parallel colour system is common. In order to obtain colour images in colour displays, either light of different sources having a different wavelength or wavelength range can be used, or light of a white light source is used and split into a number of colours. Typically three basic colours or colour ranges are used, which correspond with the three primary colours red, green and blue.
Recently the use of light emitting diodes (LEDs) as light sources in projection systems has gained popularity. Typically red, green and blue LEDs are provided to generate the red, green and blue primary colour in a projection system. In order to fulfil the high brightness requirements, often a plurality of light emitting diodes are used in the same colour light channel. The plurality of light emitting diodes for the same colour light channel then are selected to have a predetermined specific colour whereby only small spectral variations are allowed. Such small spectral variations typically are translated as a peak wavelength difference being maximally 10 nm. In this way it is tried to have sufficient colour purity. Combination of the light beams of the different light emitting diodes in a single colour beam typically is based on positioning the different light emitting diodes close together.
Due to the significant increase in the number of large screen applications over the last decade, multi-display applications such as multi-projector display systems allowing tiled-wise displaying of images have become more popular. In such applications, colour differences between different tiles significantly reduce the quality of the image. As the human eye can detect very subtle colour shifts or brightness changes, the optical non-uniformity and asymmetry introduced at the borders of tiles can produce disturbing optical effects. The latter again poses stringent requirements on colour control in displays or projection systems.